1. Field of the Invention
The present invention generally relates to a method and an apparatus for acquiring X-ray CT images of a biological body under medical examination in a helical scanning mode. More specifically, the present invention is directed to such a method/apparatus that the X-ray CT (computerized tomographic) images of the biological body such as a heart of a patient is reconstructed in synchronism with an electrocardiogram of the biological body.
2. Description of Prior Art
Various types of X-ray CT imaging apparatus have widely been utilized in the medical examination field. When a heart and blood vessels of a biological body under examination are medically examined by employing such X-ray CT imaging apparatuses, a plurality of tomographic image data thereof are continuously produced, and then either a three-dimensional surface image or an MPR (multi-planar reconstruction) image is produced based upon the plural pieces of tomographic image data.
In accordance with one conventional X-ray imaging system, the scanning operation is performed in such a manner that a couch on which a patient (i.e., biological body under medical examination) is set, is intermittently translated along its longitudinal direction every time a single piece of tomographic image data is acquired, while interruptedly projecting X-ray beams to the patient. As a consequence, a lengthy X-ray CT imaging time is necessarily required so as to obtain not only X-ray CT images, but also such a three-dimensional surface image and an MPR image.
To avoid such a lengthy X-ray image acquiring/processing operation, another X-ray imaging system so-called as "a helical scanning system" has been utilized in which while X-ray beams are continuously projected to the patient and a couch is also continuously translated along a longitudinal direction thereof. As a result, an organ of the patient, for instance, a heart and blood vessels can be medically examined at a higher speed than in the first-mentioned conventional X-ray CT imaging system. The helical scanning type X-ray imaging system is known from, for instance, U.S. Pat. No. 4,630,202, entitled "COMPUTERIZED TOMOGRAPHIC APPARATUS UTILIZING A RADIATION SOURCE" issued to Mori, on Dec. 16, 1986. In this helical scanning type X-ray imaging system, as represented in FIG. 1, a circumference of a patient "M " is continuously scanned by an X-ray detector unit (not shown in detail) and an X-ray source (not shown either) in a helical scanning mode, while a couch (not shown) on which the patent is laid is continuously translated along a direction indicated by "T ", whereby X-ray projection data "S.sub.0 " to "S.sub.3 " on the patient "M" are acquired and three-dimensional CT images thereof can be finally obtained by processing these X-ray projection data.
Very recently, such an idea has been developed in the helical scanning type X-ray imaging system that an X-ray contrast medium had been injected into the patient "M" prior to a commencement of a helical scanning operation in order to improve contrast effects of blood vessels in the X-ray images.
However, there are problems in the above-described conventional X-ray CT imaging systems that although a stationary portion of the patient "M" can be imaged with medical and photographic satisfaction, a moving imaging portion thereof such as a heart cannot be imaged as a series of "still" X-ray images. That is, a plurality of tomographic images which have been continuously acquired in the helical scanning mode, are obtained as not still images, but images containing motion artifacts. As a consequence, in particular, when a living heart of the patient "M" is medically examined by employing such a conventional helical scanning type X-ray CT imaging apparatus, it is practically difficult to acquire three-dimensional "still" images of this living heart, namely three-dimensional images containing motion artifacts.
As previously described, these conventional x-ray imaging systems have the following drawbacks. That is, since a lengthy X-ray examination time is required, various medical problems such as pain are given to patients. Moreover, even when the highspeed helical scanning type X-ray CT imaging systems are employed, a series of X-ray still images about a moving organ of a patient can be hardly acquired. Thus, the resultant three-dimensional images contain motion artifact components, resulting deterioration of image qualities thereof.
The present invention has been made in an attempt to solve the above-described various problems, and therefore has an object to provide a method capable of reconstructing X-ray CT images of a living organ of a biological body in synchronism with an electrocardiogram of this biological, thereby obtaining three-dimensional images thereof free from motion artifacts.
Another object of the present invention is to provide a helical scanning type X-ray imaging apparatus capable of obtaining three-dimensional X-ray images of a moving organ such as a heart of a patient from a series of X-ray still images thereof.